1. Field of the Invention
This invention relates to the combination of an isolation circuit and a common mode filter circuit which may be used, by way of example, to connect a computer to a local area network (LAN), and to an electrical connector which includes such a combined isolation and common mode filter circuit.
2. Description of Related Art
Electrical connectors known as modular phone receptacles or jacks have been known for many years. Although connectors of this type were originally designed for use in telephone systems, they have found wide acceptance in a variety of other contexts. For example, modular jacks are now commonly used as input/output (I/O) interface connectors for enabling computers to communicate with each other and with a variety of peripheral equipment, and in particular as connectors between a local area network (LAN) and an appropriately configured interface card.
In order to receive a corresponding modular plug, the conventional modular jack is generally made up of a socket housing which includes a plug-receiving opening, opposed top and bottom surfaces joined by opposed side surfaces extending from the opening to a back surface, and a plurality of stamped, metallic elongated contacts mounted in the housing for engaging contacts of the corresponding plug. Each contact in this type of connector includes a contact mating portion at one end extending diagonally into the socket, a vertically extending lead portion at the other end, and a horizontally extending intermediate portion between the contact mating portion and the lead portion. Generally, the lead portions of the contacts are inserted directly into openings in the interface card and soldered in place.
Because the above-described type of modular jack is often used for digital communications, the devices in which this type of connector is used have a tendency to emit high frequency radiation which can interfere with other electrical equipment. In addition, the devices are themselves vulnerable to noise or transients induced in an incoming line by external sources. While adding filtering circuitry to the interface card can often be used to solve such problems, the difficulty at designing circuitry which meets current emissions requirements as well as space considerations suggests that inclusion of filtering or transient suppression capabilities in the connector would be desirable under certain circumstances, and in particular where the cost of providing on-board filtering exceeds the cost of adding filters to the connector.
In general, prior attempts to add common mode filtering to interface connectors for LANs and similar applications have fallen into one of three categories:
1.) connectors in which the filter components are provided on a miniature circuit board fitted into or onto the connector, as described in U.S. Pat. No. 5,069,641 (Sakamoto et al.), or on circuit board traces applied directly to the connector, as described in U.S. Pat. No. 5,282,759 (Sakamoto et al.);
2.) connectors in which the connector contacts are inserted through central openings in a ferrite block which forms the inductive component of the common mode filter, as described in U.S. Pat. Nos. 4,772,224 (Briones) and 5,397,250 (Talend); and
3.) connectors in which the contacts are wrapped around the filter components, as described in U.S. Pat. Nos. 5,015,204 (Sakamoto et al.) and 5,139,442 (Sakamoto et al.).
Filters of the first type, in which the circuitry is provided on a printed circuit board, have the disadvantage that they are relative expensive in comparison with corresponding circuitry mounted on a host interface card or circuit board, due to the limited space available within the standard connector and the consequent need for miniaturization. Filters of the second and third types, on the other hand, are simpler to install and use less expensive bulk components, but have the disadvantage of failing to offer electrical isolation between input and output circuits, as a result of which the isolation circuitry must still be provided on the host circuit card.
FIGS. 1A-1C show a typical example of the third type of prior art filtering arrangement mentioned above, in which the common mode filter is formed by wrapping the connector contacts around a ferrite core, as disclosed in U.S. Pat. No. 5,015,204. The prior arrangement provides an inductive core 1, which is positioned at the rear of an RJ connector, and contacts that extend from a first end, the first end being in the form of a front mating interface portion 2 arranged to engage corresponding portions of the contacts of an RJ connector plug, to a second end which forms PCB tails 3 for insertion into openings in a printed circuit board 6. In order to form the common mode filter, the electrical schematic for which is illustrated in FIG. 1A, the contacts are wrapped around legs 4 of the inductive coil 1 to form coils 5.
In order to accommodate the filter, the conventional arrangement requires modification of the connector by constructing the connector of a lid member 8 and a base member 7 having an extension 9 which separates the interior of the connector into a plug receiving chamber 10 and a filter accommodating chamber 11. The filter is positioned in the chamber 11 by a cavity 12 provided in the base member 7, and held in place by a pressure bar spring 13 itself positioned in a positioning member 14 depending from the lid member 8 or upper half of the connector housing.
As indicated above, the disadvantage of this arrangement is that it does not/cannot accommodate both the common mode filter and isolation circuitry, and thus it has conventionally been necessary to include a transformer on the interface card, or eliminate bulk components in favor of circuit traces at the rear of the connector and a prepackaged miniature transformer of the type described, for example, in U.S. Pat. No. 5,403,207.
As will be discussed in more detail below, the present invention modifies the concept exemplified by the arrangement illustrated in FIGS. 1A-1C by including within the connector both a common mode filter and an isolating transformer, while using easily assembled bulk components.
This is basically accomplished by separating the front portion of the RJ contact structure from the portion which extends from the connector and is inserted into the host circuit board or interface card, and by utilizing a unique core and winding structure that enables assembly of the filter and transformer structures in an especially efficient manner. As a result of this structure, all of the circuitry shown in FIG. 2 can be placed within a connector rather than on the circuit board.